Method of producing boron-gold alloy foil



United States Patent 3,137,595 METHOD OF PRODUCING BORON-GOLD ALLOY FOILHubert Patalong, Pretzfeld, Upper Franconia, and Norbert Schink,Erlangen, Germany, assignors to Siemens- SchuckertwerkeAktiengesellschaft, Berlin-Siemeusstadt, Germany, a corporation ofGermany No Drawing. Filed Apr. 12, 1960, Ser. No. 21,591 Claimspriority, application Germany May 12, 1959 9 Claims. (Cl. 148-1) Ourinvention relates to a method for producing a p-doped region in bodiesconsisting of essentially monocrystalline semiconductor material,preferably silicon, by alloying onto the semiconductor body a foil ofboroncontaining gold.

This invention is based upon our discovery that the boron-containinggold required for such purposes is advantageously produced by firstintimately mixing gold powder and boron powder, then compacting themixture under pressure, thereafter tempering the pressed body at atemperature below the melting temperature of gold, subsequently meltingthe pressed and sintered body, and thereafter rolling the remeltedmaterial down to foil thickness. This discovery was made in the face ofprior views that boron cannot be melted together with gold because ittends to become expelled from the melt, as explained below.

It is known to produce an n-doped region in bodies of silicon byalloying a donor element, for example antimony, into the gold componentprior to joining that component with the silicon. It is further known toproduce a p-doped region in bodies of semiconductor material by alloyingonto the bodies a part consisting of aluminum. This method has thedisadvantage of requiring relatively high temperatures (700 C.) wherebythe lifetime of the minority charged carriers is more strongly reducedthan when alloying gold components into the semiconductor body, thelatter method being applicable at a lower alloying temperature (400 to500 0.).

Many attempts have been made toward making the favorable conditionsafforded by the alloying of dopecontaining gold also applicable for thepurpose of producing p-doped regions. It has been tried, especially, tothus introduce boron into the semiconductor material, because boronwould permit the obtainment of a high doping concentration, due to thehigh solubility of boron in silicon, the distribution coefiicient beingnear unity. However, the introduction of boron by diffusion, by a knownmethod, possesses the disadvantage of demanding extremely hightemperatures (900 to 1300 C.), thus entailing a great reduction inlife-time of the minority carriers. Since boron could not be meltedtogether with gold because it does not dissolve in gold but tends tobecome expelled from the melt, it has been proposed that solid boron beintroduced into the liquid gold-silicon alloy. This can be done, forexample, by first mechanically rolling amorphous boron in powder forminto a gold foil or by spreading the solid boron in fine distributionupon the foil, whereafter the gold foil is placed upon the semiconductorbody and the assembly is subsequently heated. Then the gold forms aliquid alloy with a portion of the semiconductor material, in whichalloy the boron penetrates down to the alloying front.

It is an object of our invention to bond a boron-containing gold foilwith a semiconductor body by alloying rather than diffusion. Moreparticularly it is an object of the invention to produceboron-containing gold, to prepare foils from such boron-containing gold,and to produce a p-doped region in semiconductor bodies by an alloyingmethod.

We have discovered that by means of the method according to ourinvention boron becomes dissolvable in gold after all. When gold powderand boron powder are intimately mixed, then kept compacted underpressure and are tempered several days at a temperature below themelting temperature of gold, a suflicient quantity of boron diffusesinto the intimately adjacent gold or vice versa, so that in this mannera mixture akin to a borongold alloy results. It has been found thatafter such processing, the boron is no longer present within the gold inrelatively coarse pieces but mainly in molecular distribution. Evenduring subsequent melting and rolling of the material down to foilthickness, no de-mixing takes place to any appreciable extent.

Consequently, it then becomes feasible to melt the tempered, pressedbody together with a further quantity of gold and/or other materials,particularly such other materials that improve the quality of the alloy.For example it has been found preferable to add slight quantities ofgallium and/or indium to the gold foil, in amounts of from 0.1 to 1%,preferably 0.3 to 0.4%. These two substances, likewise acting as orknown to be acceptors (p-doping), increase the doping concentration hereonly to a negligible extent, but they greatly facilitate wetting, andthe alloy formation, thus affording a considerably improved reliabilitywith respect to the control of the alloying method.

It has further been found favorable to admix in or incorporate with theboron-containing gold foil an additional trace content of oxygen and/ orsulfur, for example, preferably in the order of magnitude of a fewmillipercent. The quantity of oxygen may be 0.001 to 0.02%, the quantityof sulfur may preferably be 0.001 to 0.01%. This also affords improvingthe quality of the alloy. Furthermore, slight additional quantities ofbismuth, for example 0.01 to 1% and particularly 0.3 to 0.4% relative tothe weight of the gold, may be employed. The bismuth addition promotesthe occurrence of a planar alloying front. The slight n-doping actioncannot produce a detrimental effect due to the strong p-doping action ofthe boron.

The boron content of the foil may be 0.001 to 0.3%, being preferablyabout 0.1%, in respect to the gold, all percentages herein being byWeight.

A preferred embodiment of the method is as follows.

Gold powder and boron powder are intimately mixed with each other andare then subjected to pressure. The mixture of gold powder and boronpowder may contain 0.01 to 0.5% boron. An advantageous embodiment of theinvention contained 0.35% boron in the powder mixture. The finished foilthen contains about 0.3% b0- ron, because slight quantities of boronprecipitate at the external surfaces. It has been found preferable toapply the highest feasible pressure, for example in the order of 10,000atm. The preferred range of the pressures to be employed is from 10 to100,000 atmospheres, with a pressure of at least 2000 atmospheresdesirable, since higher pressures yield better results. Thereafter thecompressed and thus shaped body is tempered in vacuum or protectiveatmosphere such as nitrogen gas at about 900 C. for several days,preferably at least 48 hours. The value of 48 hours as minimum periodfor the processing time is suitable. Very good results were obtainedwith periods of 50 to hours. The tempering temperature must in any eventremain below the melting temperature of gold (1,063 C.). The temperingtemperature should be at least 825 C. with a temperature of about 900 C.having been found to be particularly favorable.

Thereafter, the boron content is determined of a portion of the pressedbody, and the other portion is melted together with a correspondinglydimensioned quantity of Suitgether with the gold. For example a portionof bismuth may be roasted, i.e., heated with ingress of air, and anotherportion of bismuth may be sulfurized, i.e., heated in pulverulent formtogether with sulfur flower to melting. After determining the oxygen andsulfur contents respectively, corresponding quantities of these twotypes of prepared bismuth are added to the tempered pressed material, ifdesired together with another portion of untreated bismuth. Thesemelting operations are preferably also performed in vacuum or underprotective gas.

We claim:

1. A method of producing boron-containing gold foil for p-doping a bodyof essentially monocrystalline silicon semiconductor material,comprising intimately mixing gold powder and boron powder,'the mixturecontaining from 0. 01 to 0.5% by'weight of boron powder, compacting themixture under a pressure between and.

100,000 atmospheres, tempering and sintering the compacted mixture at atemperature above 825 C. and below the melting point of gold for aperiod of time sufiicient to form a boron-gold alloy, subsequentlymelting the sintered, pressed boron-gold alloy, and thereafter formingthe material into a foil.

2. The method of claim 1, the tempering being carried out in nitrogengas.

3. The method of claim 1, the tempering being carried out in vacuum.

4. The method of claim 1, the tempering being carried out for at least aday, the compacting being carried out at at least 2,000 atmospherespressure.

5. The method defined in claim 1, the boron-containing gold foilhaving'from 0.001 to 0.02% of oxygen incorporated therein.

. .6. The method defined in claim 1, the boron-containing gold foilhaving from 0.001 to 0.01% of sulfur in-' corporated therein.

'7. The method defined in claim 1, the boron-containing from 0.01 to 0.5by Weight of boron powder, compacting the mixture under at least 10,000atmospheres ing gold foil having 0.01 to therein.

8. A method of producing boron-containing gold foil for p-doping a bodyof essentially monocrystalline silia con semiconductor material,comprising intimately mixing gold powder and boron powder, the mixturecontaining from 0.01 to 0.5 by weight of boron powder, compacting themixture under pressure, tempering and sintering the compacted mixture ata temperature below the melting point of gold for a period of timesuificient to form a boron-goldalloy, subsequently melting the sintered,pressed boron-gold alloy, and thereafter forming the material into afoil, the tempering being carried out for at least a day at atemperatureof at least about 900 C., the compacting being at a pressure of at least2,000 atmospheres. a j V 9. A method of producing boron-containing goldfoil for p-doping a body" of essentiallymonocrystalline siliconsemiconductor material, comprising intimately mixing gold powder andboron powder, the mixture containpressure and tempering and sinteringthe compacted mixture for at least two days at a temperature-of at least825 C. but below the melting temperature of gold, while under saidpressure, subsequently melting the pressed body, and thereafter rollingthe re -solidified material down to a foil.

References Cited in the file of this patent- Treatiseon PowderMetallurgy, Goetzel, Interscience Publishers, Inc., New York, 1950.Relied on page487.

1% of bismuth incorporated

1. A METHOD OF PRODUCING BORON-CONTAINING GOLD FOIL FOR P-DOPING A BODYOF ESSENTIALLY MONOCRYSTALLINE SILICON SEMICONDUCTOR MATERIAL,COMPRISING INTIMATELY MIXING GOLD POWDER AND BORON POWDER, THE MIXTURECONTAINING FROM 0.01 TO 0.5% BY WEIGHT OF BORON POWDER, COMPACTING THEMIXTURE UNDER A PRESSURE BETWEEN 10 AND 100,000 ATMOSPHERES, TEMPERINGAND SINTERING THE COMPACTED MIXTURE AT A TEMPERATURE ABOVE 825*C. ANDBELOW THE MELTING POINT OF GOLD FOR A PERIOD OF TIME SUFFICIENT TO FORMA BORON-GOLD ALLOY, SUBSEQUENTLY MELTING THE SINTERED, PRESSEDBORON-GOLD ALLOY, AND THEREAFTER FORMING THE MATERIAL INTO A FOIL.